1. Field of the Invention
The invention relates to improved apparatus and method for repairing a metallic portion of an article using a metal powder and a laser beam, and specifically to repairing a blisk assembly such as are found in gas turbine engines.
2. Description of the Prior Art
An aircraft gas turbine engine or jet engine draws in and compresses air with an axial flow compressor, mixes the compressed air with fuel, burns the mixture and expels the combustion product through an axial flow turbine that powers the compressor. The compressor includes a disk with blades projecting from its periphery. The disk turns rapidly on a shaft, and the curved blades draw in and compress air. In the past, the blades and the disk were structures that were manufactured separately and assembled together. More recently, the blades have been manufactured integral with the disk forming a single integral structure known as a blisk.
Blisk components manufactured by all known methods of manufacture (milling, electrochemical machining) are subject to foreign object damage (F.O.D.) of the airfoils. This can occur during the original manufacturing process or during field service of the blisk. In either case, repair of the damaged region is often desirable due to the high replacement cost of the components. To date, the only repair method demonstrated on an engine test is mechanical removal of the damaged material and essentially leaving the blisk "as-is.' While this repair method is suitable for minor airfoil edge damage, it is not acceptable for more significantly damaged areas due to imbalance problems and potential mechanical property degradation. Severe airfoil damage, as experienced by a XTC46 Core Driven Fan Stage (CDFS) damaged during testing, requires a complete replacement of airfoil material from the midspan shroud outward.
Techniques have been developed for forming a compressor blade integral with a substrate in new manufacture. One technique deposits layers of filler material on a substrate through the interaction of a laser beam. A CNC program is used to control the shape of the deposit, and a contoured buildup of suitable height is achievable through multiple layer buildups, with each layer having the desired geometry. Apparatus for performing controlled laser deposition of material on a substrate is described in U.S. Pat. No. 4,730,093, and this technique is described in U.S. Pat. No. 5,038,014 in which blades are made by welding, both of which U.S. Patents are incorporated herein by reference. While the technology described in these patents has been used to successfully repair minor damage to titanium (Ti 6-4) materials by applying the same feed or replacement material as the damaged base materials, no large scale repairs have been accomplished, and no repairs have been performed by depositing a feed or replacement material that is different than the substrate base material.
One of the problems experienced in attempts to manufacture and repair items using laser welding techniques such as described above include low integrity welds. These low integrity welds have been due to oxygen and nitrogen pickup due to loss of the argon cover over the molten weld metal, referred to as the weld puddle. Contamination due to oxygen and nitrogen causes a brittle weld repair, which is an unacceptable result for an airfoil repair.
Impact due to foreign object ingestion by a gas turbine engine could result in a failure of the repaired area, that could lead to separation of the portion of the blade above the repaired area.
Drawbacks with the system set forth in these patents include operator fatigue. Even though the system is computer controlled, an operator must visually observe the application of the powder and must override the system if there is any deviation form the computer controlled program. Furthermore, as noted in the prior art patents, the systems are sensitive and failure to override in a timely fashion can lead to an unacceptable region of repair. Also, the systems of the previous patents rely upon an argon feed to assist in carrying the powder from the reservoir to the article and to shield the pool of molten metal, and upon a short interaction time of the powder in the molten pool created by the laser. However, problems with this technique are frequent loss of the protective argon gas over the molten metal, as set forth above. Spattering of molten metal onto adjacent blades and the undesirable heating of adjacent blades from laser energy reflections from the weld pool due to the close arrangement of blades around the blisk pose additional problems.